In conventional fighter aircraft head up displays are made available to the pilot so that target and navigational data and symbols produced on a CRT may be projected onto an optical element in front of him. As a result, the data and symbols become visually superimposed with objects appearing outside the windscreen and in a single field of view.
The prior art is exemplified by U.S. Pat. No. 3,230,819 issued Jan. 25, 1966, to Noxon. FIG. 1 herein schematically illustrates the invention disclosed in Noxon wherein there is provided a combining mirror having image-producing capabilities. The mirror may be fabricated from a thin, partially silvered or dichroic parabolic mirror to which the pilot views the outside world by transmitted rays. The combining mirror collimates the image from a CRT-projected display, the virtual image theoretically located at infinity. Therefore, the rays to each of the pilot's eyes are made parallel.
FIG. 2 herein illustrates a second figure from the Noxon patent wherein the prior art system is arranged in an aircraft with the dichroic parabolic mirror positioned adjacent the windscreen and with the CRT and a lens image projector positioned immediately beside the head of the pilot. In operation, the pilot views the combining mirror and sees the navigational data and symbols (which appear on the CRT) superimposed with actual targets or other objects in his line of sight.
Although the advantages of head up displays are well recognized, a primary disadvantage of them is the introduction of parallax problems due to windscreen angular deviation of light rays from objects outside the windscreen. These problems are referred to in the literature as vergence, dipvergence, divergence, disparity, etc.
When an object is viewed by a pilot through a curved windscreen, the lines of sight from the object to the eyes of the pilot strike the windscreen at different angles of incidence and emerge nonparallel. Using a present art head up display with both eyes of the pilot at the exit pupil (or exit window), the pilot sees an object outside the windscreen at a different angular position with each eye but sees parallel or collimated symbols or reticle from a head up display that he may be required to superimpose on the object at essentially the same position with each eye. As a result, the pilot cannot avoid seeing one object and two reticles or two objects and one reticle depending on his diplopia threshold and on which set his eye-brain visual system selects.
To appreciate how this occurs, reference is made to FIG. 3 wherein a curved windscreen 10 is shown. Informational symbols relating to navigation, weapon control, etc., are projected from a CRT 12 or other visual display to an optical combining mirror 14 or "combiner." The pilot's left eye will see the reflected rays 16 from CRT 12 while his right eye sees reflected rays 18. In order to perfect superposition of the reflected CRT symbols on an object outside the windscreen, image rays 24 and 26 from an external object should pass through the windscreen and remain parallel. However, as indicated by the transmitted rays 28 and 30, this does not occur due to the different angles of incidence at windscreen points 20 and 22. A parallax condition results and the left eye of the pilot does not see the same superimposed images as his right eye. If, for example, the CRT were to generate a simple ring reticle and if the outside world presented a target to the pilot, his left and right eye images would be as shown in FIG. 4A. His brain interprets what he is seeing as either condition shown in FIG. 4B. This obviously presents serious problems to a high-speed aircraft pilot and the problem becomes exacerbated as more sleek windscreens are developed in aircraft locating the pilot farther back from a relatively thick windscreen. Decollimation of the head up display cannot be used as a solution for this problem.